1. Field of the Invention
The Present invention relates to a filter medium for molten metal which is used for filtering off solid impurities (for example, non-metallic inclusions) mixed in a molten metal, in particular, a molten aluminum or molten aluminum alloys (in this specification, they will hereunder be generically referred to as simply "molten aluminums").
2. Description of the Related Art
In general, solid impurities, in particular, non-metallic inclusions are mixed in a molten metal such as molten aluminums (molten aluminums will hereunder be described as a typical example of molten metals), therefore, if such a molten aluminums containing these inclusions is used in casting and/or rolling into a product such as a disk material without any pre-treatment, the presence of the non-metallic inclusions mixed in the molten aluminums becomes a cause of various defects such as pinholes. To prevent any occurrence of such defects, the molten aluminums are in general filtered to remove non-metallic inclusions prior to casting the same. As apparatuses for filtering the molten aluminums, there have been known a variety of apparatuses, for instance, as disclosed in Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") No. Sho 52-22327, a RMF (Rigid Media tube Filter) is used which is produced by binding a refractory grain such as electrofused alumina with an inorganic binder such as SiO.sub.2, B.sub.2 O.sub.3 and the like.
However, when the foregoing conventional filter medium is used for filtering a high purity molten aluminum used in the production of, for instance, a material for computer hard disks, or for filtering molten Al-Mg alloys used in the production of a material for cans for cooling beverages or beer, the filter medium suffers from a problem in that SiO.sub.2 and/or CaO components included in the inorganic binder are reduced into free Si and/or free Ca in the molten aluminums and they are released in the molten aluminums to thus cause contamination thereof. In addition, the molten Al-Mg alloys is active and therefore, it may erode the inorganic binder, reduce the bonding strength between refractory grains, accordingly often reduce the durability of the resulting filter medium and in the worst case, result in the breakage of the filter medium during its practical use.
To deal with these problems, filter mediums produced by using inorganic binders free of SiO.sub.2 are disclosed in, for instance, J.P. KOKOKU Nos. Hei 5-86459 and Hei 5-86460 and Japanese Un-Examined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Hei 2-34732. These filter mediums never suffer from a problem of secondary contamination of molten aluminums due to the release of free Si. However, the filter medium produced by using such an inorganic binder suffers from various problems. For instance, since the bonding strength between refractory grains is poor, a filter medium has low strength and the amount of the molten metal passed through the filter medium varies widely because it easily causes loading.
For this reason, there has been a need for the development of a filter medium which can solve the problems of the release of free Si and low bending strength. J.P. KOKAI No. Hei 5-138339 discloses a filter medium which can satisfy the foregoing requirements, which is produced by using an inorganic binder comprising a predetermined amount of SiO.sub.2 for the purpose of improving the wettability of the resulting filter medium with the molten aluminums and in which needle crystals of 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3 are precipitated.
In practicing the invention disclosed in the foregoing J.P. KOKAI No. Hei 5-138339, however, it is necessary that an inorganic binder is once melted and then the crystal length is adjusted to a specific range using a particularly designed cooling means to thus obtain needle crystals of 9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3, and moreover, 15 to 25% by weight of SiO.sub.2 and other additives such as CaO must be incorporated into the binder and accordingly, the inorganic binder is still insufficient in terms of the foregoing prevention of the release of free Si or free Ca.
To produce molten aluminums employed in various uses such as the recent disk materials whose quality is greatly affected by even slight surface defects, even fine non-metallic inclusions should be removed from the molten aluminums as much as possible and the amount of impurities dissolved in the molten aluminums after filtration should be reduced to the lowest possible level. Therefore, there has been desired for the provision of a filter medium in which the content of SiO.sub.2 and CaO components present in the inorganic binder is reduced to the lowest possible level and which has a satisfactory strength (in particular, hot strength) or the like.